There are numerous operational satellite systems currently deployed in space, and several more in the planning stages. Satellites orbit the earth at prescribed altitudes and orbit inclinations, the inclination being the angle the plane of the orbit makes with the plane of the earth's equator. Satellite orbits can be categorized in three altitude ranges: Low earth orbit (LEO) from 300 to 1500 km; medium earth orbit (MEO) from 10,000 to 20,000 km; and high earth orbit (HEO) from 35,000 km and above. Orbit altitudes and orbit inclinations are selected to achieve desired performance objectives (e.g., global access, ground resolution, orbit period, etc.), and also selected to avoid regions of high natural radiation which can damage satellite equipment.
A satellite system can be deployed as a single satellite or as a plurality or constellation of similar satellites that occupy the same spherical altitudes, and cooperate together in performing a mission. For example, many communication satellite systems operate at HEO altitudes at near-zero inclination. Their resulting 24-hour orbital periods coincide with the earth's rotational rate causing the satellite to remain fixed above a specified equatorial ground point, a feature that facilitates ground antenna pointing. The Global Positioning System (GPS) is a multi-ring 24-satellite MEO constellation that employs 20,000-km circular 12-hour orbits inclined at 55 degrees. U.S. Pat. Nos .5,551,624, 5,439,190, and 5,415,368 disclose MEO constellation of communication satellites in 10,000-km, six-hour orbits. IRIDIUM is a LEO constellation of 66 communication satellites at 785 km altitude equally distributed in six planes inclined at 86.4.degree.. When specific mission satellites, such as earth imaging satellites, are deployed in the lower end of the LEO orbit range (for example between 300 to 600 km), the orbit is referred to herein as a very low earth orbit (VLEO).
A satellite constellation may feature crosslinked communications enabling inter-constellation connectivity in a tangential or lateral direction from one satellite to another satellite around the orbital sphere. This inter-constellation connectivity allows a message to be uplinked radially from a ground site to any satellite in the constellation, then relayed tangentially from satellite to satellite around the globe and then radially downlinked to a remote receiving ground site. Communication satellite constellations are typically crosslinked to enable global connectivity. A signal from the ground may be received by a satellite through a radially directed communication uplink, retransmitted by that satellite to a second satellite through a tangentially or laterally directed crosslink, and then retransmitted by the second satellite to a ground receiver through a radially directed downlink. Some communication satellites operate primarily as relays, where messages are received from the ground and simply retransmitted back to the ground at another location.
Earth-based or space-based microwave transmission can be directed through broad beams or narrow beams depending on the antenna design and transmission frequency, and are selected to be compatible with parameters and constraints of the particular link application in terms of range, satellite-to-satellite geometry dynamics, transmitter power, data rate, weather, or other operational factors. Broad beams or wide beams are created using small omni-directional antennas, and are best suited for coarse pointing, low signal power, low-data-rate applications. Narrow beams employ larger unidirectional antennas that require more precise pointing and generally deliver higher power to the receiver necessary for high-data-rate transmission. The beam width is selected to suit the particular application and can span the range from very broad to very narrow. The corresponding communications signals may include very low-data-rate command messages, or low-data-rate voice, or very high-data-rate multi-color images. There are always design trade-offs between transmitter power, beam widths, and data rates, but generally for high-data-rate communications the beam width is narrow and the power delivered to the receiver is high, and for low-data-rate communications and broadcasting the beam width is broad and the delivered power is lower. Satellite crosslinks generally employ very narrow microwave or laser transmission beams that focus the transmitted energy on the receiving antenna. Narrow beams allow efficient use of transmitter power over longer ranges, and maximize the transmitted data rate. A disadvantage of narrow beams is that they require very accurate pointing and tracking, and often require the use of a secondary wider beam to search for and acquire the receiver target and establish the link before narrow beam transmission can be initiated. This acquisition and hand-off procedure is repetitive as communicating satellites move in and out of view and the link must be rerouted.
LEO mission satellites that monitor the earth collecting environmental records or ground images generate large quantities of data at high rates. If the collection satellite operates in the "store-and-dump" mode, the collected data is electronically stored onboard until the satellite overflies a ground read-out station, whereupon the data must be downlinked at an accelerated rate while the satellite is in view of the ground station. This method of operation requires the satellite to have ample onboard data storage and read-out capability, and suffers inherent time delays between data collection and read-out opportunities. Today, if the collection satellite operates in a "real time" relay mode, a wideband (high-data-rate) space link must be established and maintained between the data collecting mission satellites and a mission specific or partially dedicated HEO relay communication satellites. This method of operation requires precise tracking over long ranges (.about.25,000 km) using very narrow beams, and involves periodic reacquisition procedures as the satellites are eclipsed by the earth. The problems associated with long-range communications drive all aspects of the communication system design, and strongly influences the size, weight, power, and cost of the hosting satellite.
Successful link closure between communicating satellites usually requires special hardware and processes to account for the time variation of the satellite-to-satellite geometry. The determination of precise antenna pointing angles and rates, range monitoring, Doppler shift correction, acquisition and hand-off procedures are typical processes in achieving and maintaining satisfactory link closure. For example, when a VLEO satellite communicates with a constellation of HEO communications satellites, the link must be reestablished with a second HEO satellite before the currently-linked HEO satellite passes out of view of the VLEO satellite. The acquisition and hand-off to the second HEO satellite must be accomplished over extremely long ranges and in rapid fashion to avoid gaps in communications. For another example, a constellation may include several satellites distributed in multiple orbit planes (sometimes referred to as rings of co-planar satellites), each ring defined by the orbit altitude and the orientation of its orbit plane. The constellation may exhibit inter-ring and intra-ring crosslink communications creating time varying Doppler shifts in the received carrier frequency especially during intra-ring communications. The onboard communication subsystem is usually required to retransmit received range tones which can then be used to appropriately adjust the transmission for Doppler correction.
The rapidly changing relative geometry can be even more pronounced when communicating between two LEO satellites in different orbits and different orbit planes. At LEO, the severity of the relative geometry problem is largely dependent on the degree of alignment of the two orbit planes. When the planes of the orbits of two satellites are at a large angle to each other, the relative motion between the two satellites moving in their respective planes can be extremely large depending on the degree of planar misalignment. The two orbit planes are constantly rotating relative to the stars and relative to each other depending on the parameters of the two orbits. Even when the orbit planes are initially aligned, the orbit planes of unmatched orbits will slowly drift out of alignment due to the difference in their planar rotations. This well-known orbital perturbation effect that is caused by the earth's equatorial bulge is called nodal regression of the orbit plane, or the rotation of the orbit plane, relative to the stars, about the earth's polar axis. For LEO orbits, this rotational rate may be only a degree or two per day; however, in a relatively short time, it can result in severe misalignment of the planes of communicating LEO satellites with mismatched nodal regressions. The orbit plane rotational rate is a function of the orbit altitude and the inclination of the orbit plane. With proper selection of inclination, it is possible to match the nodal regression rates of two LEO orbits that are at different altitudes. The following equation would be used for calculating the correct inclination that results in matched nodal regression of circular orbits: ##EQU1## where i.sub.1 =inclination of orbit #1
i.sub.2 =inclination of orbit #2 PA1 h.sub.1 =altitude of orbit #1 PA1 h.sub.2 =altitude of orbit #2 PA1 RE=earth radius
If the satellite's orbit has an inclination angle of 90.degree., the resulting nodal regression rate is zero and the matching orbit would also have a 90.degree. inclination.
There are a growing number of existing and planned LEO constellations of communication satellites designed to provide a variety of mobile communication services to terrestrial users. These constellations, referred to as Big LEO satellite communication systems, consist of dozens (sometimes hundreds) of crosslinked satellites distributed uniformly about the globe in multiple planes or rings at a common altitude. In some cases, each ring may include dozens of satellites equally spaced along its circumference. Big LEO systems use large constellations of satellites to provide telecommunications services at fixed and graduated data rates for users on the ground. For example, the IRIDIUM system currently features 66 satellites distributed in six planes or rings and delivers low-data-rate mobile telephone service to subscribers in North America. For another example, Teledesic is a planned Big LEO system that will provide broadband (high-data-rate) global service suitable for rapid transmission of imagery and other very large data records. An early version of the planned Teledesic constellation called for 288 satellites uniformly distributed in 12 rings about the globe.
The need to provide uniform regional or global coverage with the fewest satellites drives the Big LEO orbits to the highest altitudes practical in the LEO range. (The earth's natural radiation environment constrains the upper bounds of the LEO altitudes.) Earth monitoring mission satellites, on the other hand, tend to fly at much lower (VLEO) altitudes in order to minimize sensor aperture (payload size and weight) needed to achieve the desired ground resolution. One or more VLEO mission satellites orbiting in the celestial presence of a Teledesic-like Big LEO system would suggest an architecture with multiple rings of Big LEO communication satellites criss-crossing aperiodically above the lower altitude mission satellites.
The problems faced by the mission satellite in delivering its data retrievals to a ground site are formidable. Many mission satellites today store the collected data onboard until they overfly the ground station, and then read out the data at an accelerated rate while the station remains in view. Store and dump operations as described require adequate onboard data storage capacity, and a sophisticated data read-out capability. These operations are also disadvantaged by inherent time delays between data collection and delivery. Other mission satellites today transmit large data records to the ground site, in real time, using a dedicated HEO relay satellite system. This operation requires long-range communication technologies involving precise pointing and tracking with large narrow beam antennas and high power transmitters; in addition, the cost of a dedicated HEO relay may be as much or more than the mission satellite. Communications between a LEO mission satellite and a non-co-orbital LEO communication satellite requires overcoming a myriad of problems dealing with the rapidly varying relative geometry and a high nonperiodic acquisition and hand-off frequency. These and other disadvantages are solved or reduced using the invention.